Hmm. Not sure about 'horses' Karen.
The second letter could possibly be an 'o' instead of an 'a', but I can't see the fourth letter being an 's' instead of an 'n'.
The writer makes all 's's I can see pretty much the same.
'Harness' is usually written with 2 's's, but then there are some irregular spelling of other words too in the letter.
There are lots of irregular spellings and it reads as though he wrote it in a hurry.
I sat looking at the word for a long time, as the letters are a little strangely formed, but I still think it could be "horses", though, as there are later references to horses and it would certainly make sense concerning the context.
Lower down in the letter, he writes "got mounted and cantered away". The end of the word has been amended, but it can only be "cantered", meaning there is no "s" in the word. What is definitely an "r" (but looks like an "s") is extremely similar to what would be the "r" in "horses" in the first section.
Best regards,
Karen
Edit: I'm harping on about the letter "r" and you were actually talking about "s". Oops.

Having had (yet) another look, I disagree a wee bit about his "s" being consistent - it varies wildly. Sometimes a clear "s", sometimes almost a circle, sometimes almost an "8" and sometimes just a blob...
Edit II: I've just noticed that the best comparison is in the same sentence: ...
orders to unhook...
